Method and System for Soil and Groundwater In Situ Redox Potential (Eh) Logging

Abstract

Soil and groundwater redox dictate the microbiological processes governing the subsurface, which are of utmost importance for agriculture, soil remediation, and groundwater protection. Despite its relevance, there is no scientific consensus on whether it can be measured. We propose a method and system for in situ continuous logging of redox potential (Eh) with depth by measuring the voltage difference between a reference electrode at the surface and a Pt electrode embedded in a probe that can penetrate the soil down to 50 m using a direct push (DP) drilling rig. Results from 27 unconsolidated geological settings in Denmark reveal redox zonation with depth, indicating which chemical reactions can be expected in oxidized, transition, and reduced sections, with a suggested redox interface. We found all interfaces below the groundwater table (GWT). 51% of the variation in the redox interface is explained by variables other than distance to the GWT. The size of the transition section might indicate a combination of GWT fluctuations and the size of the capillary fringe. We reviewed the ISO standard for soil Eh measurement. Knowing the depth of the redox interface is important for understanding and managing subsurface environmental processes.

Redox potential (Eh) is measured with depth in soil and groundwater to identify where significant changes in electron activity occur and, thus, which chemical reactions can be expected.